You are deputy leader. Your own party has suffered a terrible recent fall in popularity. (Indeed, these days it's difficult to find anybody who actually likes you.) Oh, and you are a committed Euro-integrationist and liberal who favours the colour yellow.

Westerwelle also has another job: he is German foreign minister. Eighteen months after he took on the role it is difficult to find anybody in Berlin who has positive things to say about him. German foreign affairs pundits are politely scathing. ("He doesn't have a mental habit of digging deep," one says. "He's squashed between Brussels and Merkel's chancellery," adds another. He has a "short attention span" and isn't "detail-oriented", complains a third.)

All point to Westerwelle's lack of foreign policy experience. Certainly, he is far more comfortable weighing in on domestic and economic issues. And there are inevitable and unflattering comparisons with Hans-Dietrich Genscher, Germany's legendary liberal foreign minister, who served for two decades in Helmut Kohl's Christian Democrat (CDU)-FDP government – the same configuration as today's centre-right coalition – and seemed to personify noble-minded European statesmanship.

And then there are Westerwelle's alleged personal deficiencies – he is prickly, patronising and supremely thin-skinned, according to those who are not fans. Last year's leaked secret US state department cables had some nasty things to say about Merkel, Germany's chancellor since 2005. US diplomats in Berlin described her as "risk-averse and seldom creative".

But they were harsher about Westerwelle, painting him as vain, incompetent and a burden on the German-US partnership. To add insult to injury, it turned out that the head of Westerwelle's private office had been surreptitiously passing details of post-election coalition negotiations with the CDU to the US embassy in Berlin. The mole, Helmut Metzner, was unmasked and sacked. He has subsequently said he did nothing wrong.

Some, though, think that after a disappointing start Westerwelle is doing better. Georg Mascolo, editor-in-chief of Der Spiegel, the German news magazine, says Westerwelle – unlike William Hague – has had a "good" Arab revolution. Westerwelle recently visited Tahrir Square in Cairo. He has spoken passionately in favour of freedom and democracy in the Muslim world, seemingly discovering an international issue that genuinely engages and inspires him.

Westerwelle and Merkel this week made clear they oppose any western military intervention in Libya, even as pro-Gaddafi forces pummel the rebel stronghold of Benghazi, and a massacre looms. During a meeting of G8 foreign ministers, Germany strongly resisted French and British-led attempts to impose a no-fly zone on Libya. Westerwelle said he was "very sceptical" about the prospects of success of military engagement. He warned of a slippery slope, adding: "I don't want German soldiers to be sucked into a Libyan war."

But as Constanze Stelzenmüller, senior transatlantic fellow at the German Marshall Fund of the United States, points out, reunified Germany has jettisoned its powerful postwar taboo against the use of force. In March 1999 Gerhard Schröder, then chancellor, took the momentous decision to join Nato's air campaign against Serbia, describing it as the moment when German foreign policy "became normal". (Schröder, of course, later thumbed his nose at George Bush's decision to invade Iraq.)

Since then Germany's Bundeswehr has played a role in international security missions. Berlin's foreign policy is increasingly active. More than 6,500 German troops are involved in overseas deployments from Afghanistan (they are up in the relatively quiet north, but have still suffered more than 50 casualties) to the Balkans to Congo. The German navy has sent its warships off the coast of Somalia, and in 2006 to the waters off Lebanon.

"The return of the military option to Germany's foreign policy toolkit was a necessary – if long deferred and reluctantly undertaken – step towards normalising the country's international profile," Stelzenmüller says. This does not mean – as Libya shows – that Germany is an automatic proponent of military force. But Germans, once Europe's most guilt-ridden pacifists, are now prepared to support the military option, provided the circumstances are exceptional and morally compelling.

At home, Merkel seems set to go on and on as chancellor. But the fate of Westerwelle – the deputy Kanzler – remains less certain.

Since Germany's last federal election in 2009 the FDP's poll ratings have come tumbling down, Nick Clegg-style, from 14.6% to around the 5% mark. The figure is significant: all German political parties have to pass a 5% constitutional barrier to get into parliament. Less than this means extinction.

"He's facing huge pressure from inside his party," says Alexander Rahr, an expert at the German Council on Foreign Relations. Rahr says that if the pro-business FDP does badly in two regional elections this month, Westerwelle will face internal calls for him to relinquish one of his two current jobs as party leader and foreign minister. Juggling both has become increasingly difficult.

Westerwelle's personal relationship with Merkel is a mystery. They share hobbies: both are keen opera buffs; they attend the annual Wagner festival in Bayreuth. In 2004 Westerwelle came out as gay when he arrived at Merkel's 50th birthday party with his partner, Michael Mronz. (Mronz, a sports promoter, has controversially been included as a member of Westerwelle's German trade delegations abroad.) But it is clear that, like all German chancellors, Merkel keeps a tight grip on foreign policy, leaving Westerwelle in charge of Africa and minor European issues.

The comparison with Clegg is alluring – junior partners, struggling to make their mark, in unhappy coalition marriages. But there are certain differences, with Westerwelle apparently no match for the polylingual Liberal Democrat leader. In September 2009 a BBC reporter asked Westerwelle how he planned to change Germany's foreign policy. The reporter also asked Westerwelle to answer in English. Westerwelle said the reporter should learn German instead, further evidence for his enemies of his gaucheness. He said in German: "We could meet for a fabulous tea outside of a press conference and speak only English … but this is Germany here." According to Stelzenmüller: "It was cringe-orama, as the Americans would say."